Germanium Made Laser-Compatible

A manufacturing technique that alters the optical properties of germanium — an element incompatible with lasers — has rendered the semiconductor laser-compatible through high tensile strain. The discovery could enable microprocessor components to communicate using light, making the computers of the future faster and more efficient.

To improve computer performance, computer chips have been made smaller and more densely packed, but this method eventually will hit a brick wall, according to researchers from ETH Zurich, the Paul Scherrer Institute (PSI) of Villigen and the Politecnico di Milano of Como, Italy.

“In order to increase performance and speed further, the individual components need to be linked more closely and communicate with each other more efficiently,” said Martin Süess, a doctoral student at the Laboratory for Nanometallurgy at ETH Zurich. This requires new transmission paths that are faster than current methods, which transmit signals via electricity and copper cables.

“The way to go in future is light,” said Richard Geiger, a doctoral student at PSI’s Laboratory for Micro- and Nanotechnology and ETH Zurich’s Institute for Quantum Electronics.

To transfer data using light, light sources that are small enough to fit onto a chip and react well to silicon are needed. Silicon, the base material for all computer chips, is on its own not suitable for the construction of laser light. This is why the researchers have looked to making germanium laser-compatible.

“Germanium is perfectly compatible with silicon and already used in the computer industry in the production of silicon chips,” Geiger said. If it is possible to build tiny lasers out of germanium using the new method, a system change is within reach, the investigators said.

Light-emitting bridges of germanium can be used for communication between microprocessors, an international team of researchers from ETH Zurich, Paul Scherrer Institute and Politecnico di Milano has found. Courtesy of Hans Sigg, PSI.

To make germanium into a laser-compatible, stretched form using the technique, the investigators exploited the slight tension generated in germanium when it evaporates on silicon, strengthening this prestrain with so-called microbridges. The team scored exposed germanium strips, which remain attached to the silicon layer at both ends, in the middle on both sides. The two halves remain connected solely by an extremely narrow bridge, which is precisely where, for physical reasons, the germanium’s strain grows so intense that it becomes laser-compatible.

“The tensile strain exerted on the germanium is comparable to the force exerted on a pencil as two lorries pull upon it in opposite directions,” said Hans Sigg, the project manager at PSI. The material properties change because the individual atoms move apart a little through the expansion of the material, which enables the electrons to reach energy levels that are favorable for the generation of photons.

“With a strain of 3 percent, the material emits around 25 times more photons than in a relaxed state,” Süess said.

“That’s enough to build lasers with,” Geiger said.

The team is now using the method — detailed in Nature Photonics (doi: 10.1038/nphoton.2013.67) — to construct a germanium laser.

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